Variable inductance device



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United States Patent VARIABLE INDUCTANCE DEVICE Wayne H. Graves and John A. Melichar, Cedar Rapids, Iowa, assignors to Collins Radio Company, Cedar Rapids, Iowa, a corporation of Iowa Application November 28, 1952, Serial No. 323,056

Claims. (Cl. 333-24) The present invention relates in general to variable inductance devices and more particularly to a device for accurately varying an inductance in the very high and ultra high frequency ranges.

One object of the present invention is to obtain a means for precisely adjusting an inductance at high frequencies.

Still another object is to provide means for making fine frequency adjustments of circuits used at upper radio frequencies.

The present invention provides an economical and easy way to obtain exact adjustment of coupling between electrical circuits used at high frequencies.

Yet another object is to obtain a simple way of trimming an inductance. Heretofore, to adjust the inductance of a bar or rod, it has been necessary to laboriously file it to size by trial and error. This invention provides a quick and accurate way for adjusting an inductance.

Another objectis to provide for fine adjustment of cut-off frequencies of high frequency filters containing inductances.

A feature of this invention is to provide a sleeve that may be moved longitudinally along a shaft to vary inductance.

Other objects, features and advantages will be apparent from the following specifications and drawings, in which:

Figure 1 is a sectional view taken on line 11 of Figure 2;

Figure 2 is a front elevational view;

Figure 3 is a partial top view;

Figure 4 is a schematic diagram of the device shown in Figure 2;

Figure 5 is a perspective view of a circuit with my invention used therein;

Figure 6 is an enlarged partial cross-section of the invention;

Figure 7 is a schematic diagram of the device shown in Figure 5;

Figure 8 is a graph of'the frequency versus current respouse in an output coupled circuit; and

Figure 9 is a graph showing variation of transmitted energy with change of coupling inductance.

Electrical conducting bodies often will act in a totally different manner at high frequencies than at low fre- The high frequency reactions are often unpredictable'and may seem unrelated to the low frequency reactions. For example, a straight bar, which has almost no resistance or reactance at low frequencies, is essentially an inductive reactance at high frequencies.

The present invention is concerned with high frequency phenomena. The term high frequency is used in this specification to mean frequencies in the very high or ultra .high frequency ranges.

Figures 1, 2 and 3 illustrate one structure of the present'invention and Figure 4 illustrates the electrical equivalent at high frequencies.

A frame member 10 is made of conducting material and fixedly supports an inductance shaft 11 at point A. In-

ductance shaft 11 is threaded throughout its length and an inductance sleeve 12 is threadedly received thereon. Inductance sleeve 12 is threaded internally throughout its length and is notched across its top. Inductance shaft 11 makes electrical contact with inductance sleeve 12 along their mating threaded portions. Both shaftll and sleeve 12 are of conducting material. When sleeve 12 is rotated, it moves longitudinally along shaft 11. The exterior cylindrical surface of inductance sleeve 12 is smooth and may be silvered to increase the conductivity.

The exterior surface of inductance sleeve 12 is in sliding electrical contact with wiper fingers 13 at B. (See Figure 1.) The fingers are mounted on finger support 16 by rivets 17.

Finger support 16 is supported by an inductance bar 19 to which it is fastened. The inductance bar 19 is formed into a hollow channel as shown and has an opening at either side through which sleeve 12 extends. Bar 19 is not in direct contact with sleeve 12, but they are connected electrically by fingers 13 and finger support 16.

As shown in Figure 2, bar 19 isfixed to an insulating support 21 by screw 22. Support 21 is also fastened to frame 10 by screw 23, Bar 19 therefore is parallel to frame 10 and extends on both sides of sleeve 12.

A high frequency voltage source 24 is connected to the end of inductance bar 19 at C and to the frame 10 at D. (See Figure 2.) During one cycle, electrical current will pass from the source 24 to the inductance bar 19 at C. It then will go to finger support 16, divide between fingers 13, and pass to sleeve 12 at B where fingers 13 contact sleeve 12.

It will then pass down the peripheral portion of sleeve 12 into shaft 11 along their threaded points of contact at the botom end of sleeve 12 at E. The current then will go down the peripheral portion of shaft 11 into frame 10. It will pass through frame 10 and finally will leave at D, passing back to voltage source 24.

It will be noted that the current is in the V. H. F. and U. H. F. ranges. Skin effect is very pronounced at these frequencies. Therefore, the current will flow almost entirely along the outside surfaces of bar 19, finger support 16, fingers 13, sleeve 12, shaft 11 and frame 10. The current will flow along the full length of the peripheral portion of sleeve 12 before it enters shaft 11 at E. At a frequency of 400 megacycles, for example, 99% of the current flowing down a copper cylindrical rod one centimeter in diameter will flow within of a centimeter depth from the surface.

It will be observed that the distance between fingers 13 at B and the frame 10 at A is constant, no matter how sleeve 12 is adjusted. Therefore, the length of current flow along sleeve 12 and shaft 11 between B and A must be constant, no matter how sleeve 12 is adjusted as long as it remains in the circuit. Also the length of current flow between C and B and between A and D must remain constant because their dimensions are fixed.

Since the length of current flow between B and A must be constant, regardless of the position of sleeve 12 on shaft 11, it will be observed that this length is made up of the arithmetical sum of the effective electrical length of sleeve 12 and shaft '11. As used in this specification, the term effective electrical length means t-hat'part of the length of a structural member which carries a con siderable amount of current and excludes that part .of the structural length which carries little or no current.

The effective electrical length of sleeve 12 is BE which changes as it moves upward or downward over shaft 11. The effective electrical length of shaft 11 is the remaining length EA. As stated above, the current flows from B down sleeve 12 through its peripheral portion ,to its bottom end before it enters shaft 11 at B,

One electrical characteristic of a conductive cylinder,

that conducts in the direction of its axis, is that its inductance is a function of its diameter. Where two cylinders are of equal length and the first is of larger diameter than the second, the inductance of the first is less than the inductance of the second. At high frequencies there is a greater diiference in inductance between them than at low frequencies due to skin effect.

As the electrical length of sleeve 12 is increased, the electrical length of shaft 11 is decreased by the same amount. The decreased length of shaft 11 is replaced by the increased length of the larger diameter sleeve 12.

By this replacement, the smaller diameter of shaft 11 is replaced by the larger diameter of sleeve 12. This replacement results in a decrease in inductance between B and A because of the larger effective diameter of cylinder 12. Since the sleeve 12 can be adjusted over small movements with respect to shaft 11 due to their threaded relationship, an exceedingly line and precise inductance adjustment can be obtained. Since the inductance per unit length of sleeve 12 is less than the inductance per unit length of shaft 11, it is seen how the effective inductance between B and A changes as the sleeve 12 is moved.

Figure 4- is a schematic diagram illustrating the electrical efiect of the structure of Figure 2 at high frequencies. Points A, B, C, D and E of Figure 4 electrically correspond to points A, B, C, D and E of Figure 2.

If a high frequency voltage source 24 is connected as shown in Figure 2 at points C and D, there is a completed electrical circuit around CB-EA-D. If suitable output leads are connected at B and A, an output may be removed which is designated as tap 1 in Figures 2 and 4. As the sleeve 12 is adjusted longitudinally, the voltage and phase at tap 1 vary in a precisely controlled fashion. The sleeve 12 and shaft 11 then combine to act as a controlled inductance element. Any suitable output circuit 27 may be attached across tap 1 through a switch 25. Switch has two other tap positions which will be used as explained below.

The variable inductance BA will be in series with that part of inductance bar 19 between C and B. If inductance CB is a large value compared with inductance BA, the latter can be considered a trimmer'inductance for the former. This combination is especially useful where the manufacturing process is unable to keep the value of fixed inductances within desired limits. Then, in order to avoid repeated cutting of an inductance bar to obtain an exact value, a variable trimmer inductance such as BA .can be inserted in series, and quick and precise adjustment can be obtained. Sleeve 12 can then be fastened securely in place by a locking pin 26. The series combination of inductances CB and BA will then be the exact inductance desired. This is shown schematically in Figure 4. Suitable instruments which are not part of this invention may be used to measure the inductance. Tap 2 may be used to switch off the output circuit 27.

If suitable leads are connected across points N and A, there will be a voltage across them deliverable to output circuit 27 from tap 3 of switch 25. See Figures 2 and 4. In this arrangement current passes from the source 24 through the whole length of inductance bar 19 to an output circuit 27 attached across NA and returns to source 24 via frame 10. Current also passes from source 24 through variable inductance BA in parallel with the output circuit 27 and also returns to source 24 via frame 10. The bar 19 will have inductance CB on one side of B and inductance BN on the other side. It will be observed that the inductance combination of bar 19, sleeve 12 and shaft 11 form a T-network. Such a T-network will be a coupling between the source 24 and any output circuit 27 across NA. The coupling inductance will be the variable inductance BA. An advantage is that the coupling between the circuits can be adjusted precisely by movement of sleeve 12 on shaft 11 which increases or decreases the coupling as desired.

The effect of various degrees of coupling is shown in Figures 8 and 9, when the source circuit and output circuit are both resonant. In Figure 8 the desired degree of coupling is designated at Z. Heretofore, no easy method of obtaining a desired degree of coupling was available at high frequencies and large tolerances for coupling were necessary. However, by use of the present invention, the exact degree of coupling can be easily obtained by adjustment of sleeve 12 to obtain an exact value of Z. Figure 9 shows how energy transmitted from the source to the output may be varied by changing the inductive coupling.

Another type of adjustable coupling is shown in Figure 5. The coupling is by means of a transformer, which insulates the source from the output circuit. The apparatus is mounted on a base plate 40 made of insulating material.

A stator support 41 is mounted perpendicular to base plate 40. Another stator support 4-2 is also mounted on base plate 40, and is parallel to stator support 41.

An L-shaped loop support 43 is fastened to stator support 4-1 above base plate 49 with member 44 of loop support 63 extending inwardly perpendicular to stator support 41. A Z-shaped loop support 46 is fastened to stator support 42.

End members 47 and 48 of loop support 46 extend perpendicular to stator support 42 and in opposite directions with the member 47 extending inwardly in the same plane as the member 44 of L-shaped loop support. Member 48 extends outwardly in another plane. Both loop supports electrically contact their respective stator supports.

A hairpin shaped tuning loop 49 forms the primary of a transformer and has a threaded leg 51 and a nonthreaded leg 50. The bottom of non-threaded leg 50 is fastened to the inward extending member 44 of loop support 4-3. The threaded tuning loop leg 51 is fastened at its bottom to the inward extending member 47 of loop support 46. Tuning loop is parallel to base plate 40. Both legs electrically contact their respective tuning loop supports.

An inductance sleeve 52 threadedly is received on leg 51. As the inductance sleeve 52 is rotated, it moves axially along leg 51. Sleeve 52 has a smooth external cylindrical surface which may be silvered for maximum conductivity. A locking screw 53 is inserted radially in the wall of sleeve 52 which may be used to lock sleeve 52. The surface of sleeve 52 is in sliding electrical contact with a finger 54 which is fastened in electrical contact to the outwardly extending member 48. This is best observed in Figure 6. 1

A stator plate 56 is mounted to an edge of stator support 41 and extends in the same plane. A second stator plate 57 is mounted to stator support 42 parallel to stator plate 56.

A bearing support 58 is mounted upright on base plate 40. A hearing 59 is supported by hearing support 58. On the other side of base plate 40, another bearing support 61 is mounted and supports a bearing 62 in axial alignment with bearing 59.

A condenser shaft 63 extends through bearings 59 and 62. Shaft 63 is made of an insulating material. A rotor plate support sleeve 64 made of a conducting material is centered and fixed on shaft 63. Rotor plates 66 are fixed perpendicularly to sleeve 64. Rotor plates 66 electrically contact support sleeve 64. The rotor plates 66 rotates as shaft 63 is turned. They pass in parallel relation to stator plates 56 and 57, thereby varying the capacitance between them through the air dielectric.

An output loop 67 of conducting material is mounted on base plate 40 below and parallel to tuning loop 49. Any suitable output circuit 68 is connected across the output terminals X and Y of the output loop 67.

If a high frequency voltage source 69 is connected across, for example, points A and B, and an output circuit 68 is connected at points X and Y, the device illustrated in Figure 5 will operate as a transformer.

During one cycle of current flow, the high frequency current will pass from source 69 to point B. Part of the current will go to the right of point B to point A and return to the source 69. The remaining current will go to the left of point B around the curve of the tuning loop 49 to point C. It will be remembered, as explained above, that at high frequencies, current flows for the most part in the peripheral regions of a rod due to the phenomena known in the art as skin effect.

The current therefore will travel along tuning loop parts BA and BC in the peripheral region of the rod. When the current reaches point C, the current will pass to the peripheral regions of inductance sleeve 52. Therefore, the current will leave the tuning loop leg 51 at point C because of the high frequency skin effect and practically no current will travel down that threaded part of leg 51 which is internal to sleeve 52 because of its much higher impedance. By way of contrast, at low frequencies much of the current would flow through leg 51 rather than pass to sleeve 52 because of the contact resistance between threads. This would prevent sleeve 52 from having any appreciable inductive characteristic at low frequencies.

As the high frequency current travels down the periphery of inductance sleeve 52, it will reach point D where finger 54 is in sliding electrical contact with sleeve 52. It will be conducted from sleeve 52 to finger 54. Since practically no current exists in the leg 51 beyond point C, almost no current will be conducted to point E.

The current will pass from finger 54 to the outwardly extending member 48 of loop support 46. The current will then pass from loop support 46 to stator support 42 and to condenser stator plate 57.

Alternating current is considered to flow through a condenser. This is a well known concept in the art and requires no further explanation. Current will flow from stator plate 57 to the two adjacent rotor plates 66. It then will flow down rotor plate support sleeve 64 to the opposite rotor plates 66. The current will flow to the adjacent stator plate 56, and will pass to stator support 41. It will then travel from stator support 41 to loop support 43, and back to the source 69 through the connection at point A.

At the beginning of the above described current flow cycle, it was stated that the current divided at B and part flowed to point A and back to the source. Although this would be a short circuit at low frequencies, it is not at high frequencies. It Will be recalled that a straight rod has a large inductive reactance at high frequencies. The rod AB will act as an inductance and will form in conjunction with the reactance of loop rod BC and sleeve portion CD, as the primary of an auto transformer. See Figure 7 for an illustration of the auto transformer schematic between points A, B, C and D. Rod portion BC and sleeve portion CD will form the secondary of the auto transformer.

That part of sleeve 52 and loop leg 51 which is between points DE has little effect because only a very small, if any, current will flow through DE.

The loop BCD is also the primary of an isolation transformer. The secondary is output loop 67. The loops are spaced parallel but are not in electrical contact. Primary BCD is in close proximity to output loop 67 and there will be mutual inductance between them. Therefore alternating current flowing through portions BCD will induce a voltage in output loop 67. This induced voltage will cause current to flow in the output circuit 67.

The voltage induced in output loop 67 will vary with the current flowing through the primary tuning loop portion BCD. Control of this current will control the coupling between the primary tuning loop 49 and secondary output loop 67.

Longitudinal movement of inductance sleeve 52 along tuning loop leg 51 will vary the inductance of portion BCD which controls the current through it. The inductance of the larger diameter sleeve 52 is less per unit length than is the tuning loop 51 which has a smaller diameter. If sleeve 52 is moved longitudinally toward point E, its effective inductance is decreased. This is because the distance between C and D is decreased, which is the effective length of sleeve 52. At the same time the length BC increases which increases the effective inductance. It will be observed that the increase in length of BC is equal to the decrease in length of CD. Therefore as the length of CD which has low inductance is replaced by the smaller diameter, a net increase in the total inductance over tuning lop portion BCD occurs. If sleeve 52. is moved longitudinally in the opposite direction the net inductance will decrease. The movement of sleeve 52 therefore varies the transformer coupling. It can be locked in any position by tightening locking screw 53 or by soldering fingers 54 to sleeve 52.

Figure 7 is an electrical schematic equivalent of the apparatus of Figure 5. Figures 8 and 9 also show the general coupling characteristics between a tuned source circuit and tuned output circuit for the isolating transformer shown in Figure 5, as Well as for the T-coupling shown in Figure 2.

Because of the threaded connection between tuning loop 49 and sleeve 52, a micrometric longitudinal adjustment can be obtained by turning sleeve 52. However, if sleeve 52 were in sliding contact with tuning loop leg 51, the same adjustment could be obtained, but it would lack the precision of movement of the threaded device.

While we have shown and described certain embodiments of our invention, it is to be understood that it is capable of many modifications. Changes, therefore, in the construction and arrangement may be made without departing from the spirit and scope of the invention as disclosed.

We claim:

1. A network having an adjustable input and output relationship at high frequencies comprising a conducting frame member, a bar member of conducting material supported insulatingly from the frame member and formed with an opening, a shaft of conducting material fixed at one end to the frame member, a sleeve of conducting material threadedly received on said shaft and situated through the opening in said bar member, contact means supported by said bar member and slideably engaging the surface of said sleeve, input and outputs provided between opposite adjacent portions of said bar member and frame member, and the input and output impedances being adjustable by varying the longitudinal position of said sleeve on said shaft.

2. A trimmer inductance at high frequencies comprising a supporting member of conducting material, a shaft of conducting material fixed conductively at one end to said supporting member, a sleeve of conducting material having a smooth outer surface and slideably and conductively received over said shaft at its end opposite said supporting member, a wiper contact supported with a fixed space relationship from said supporting member, said wiper contact slideably and electrically engaging the outer surface of said sleeve, whereby the inductance between the supporting member and wiper contact is varied by adjusting the longitudinal position of said sleeve on said shaft.

3-. Inductance trimming means for a high frequency tuned circuit including a hairpin loop of conducting material as the inductance element of said tuned circuit, a sleeve member of conducting material slideably received over a portion of said loop, a contacting finger of conducting material supported at a fixed position relative to said loop, the contacting finger slideably and electrically engaging the outer surface of said sleeve, and connecting points for the loop provided at the contacting finger and at one end of the loop, whereby the inductance of the loop is variable by changing the position of said sleeve relative to said loop.

4. Finely variable inductance means utilized as a coupling member for a filter operable at ultra-high frequencies'to control the selectivity of said filter, including a pair of supporting means of conducting material associated With said filter connecting to opposite sides of said variable inductance means, said supporting means having a fixed space relationship, said variable inductance means comprising a pair of different diameter conducting rod-like members with one of said members telescopically receivable in the other member, said one member fixed to one of said supporting means, and said other member slideably contacting said other supporting means, whereby the mean diameter of said pair of conducting members between said supporting means is controlled by the relative telescopic positions of said conducting members to control the selectivity of said filter.

5. A T-network having an adjustable input and output impedance relationship at ultra-high frequencies comprising a pair of conducting members supported insulatingly from each other, a conducting shaft fixed mechanically and electrically at one end to one of said members and extended toward the other member, a connection point at the junction of said conducting shaft and said one member, a conducting sleeve slideably received over the extended end of the shaft and situated adjacent to said other member between its ends, a wiper contact supported by said other member between its ends and slideably contacting said sleeve, input terminals of said T-network provided by said connection point and one end of said other member, a terminal point on said one member adjacent the other end of said other member, output terminals of said T-network provided by said other end of said other member and said terminal point, and the input and output impedance relationship between the input and output terminals being controlled by the longitudinal position of said sleeve on said shaft.

Reterenees Cited in the file of this patent UNITED STATES PATENTS 

